1. Field of the Invention
This invention relates to an apparatus and method of producing high purity lithium metal and the like by fused mixed salt electrolysis. More particularly this invention relates to a skimmer apparatus and method of use which allows for the efficient, economical and safe separation of the molten lithium metal and the like from the molten electrolyte.
2. Description of the Related Art
In commercial practice, lithium metal is produced by the electrolysis of a molten mixture of lithium chloride and other metal chlorides which reduce the melting point of lithium chloride. Suitable electrolytic cells have no diaphragm. Such cells are made of steel, a steel cathode, a graphite anode and have no internal lining. The molten lithium metal accumulates on the surface of the molten salts and is skimmed from said surface by means of a skimming ladle or may be withdrawn by elevators. As chlorine gas is evolved it is withdrawn from the cell. The liquid metal may be oxidized and nitrided by exposure to air. Published European Patent Publication No. 107 521 discloses a process for the continuous production of lithium metal by an electrolysis of lithium chloride contained in a molten salt mixture an electrolytic cell comprising a cylindrical steel cathode, which has been inserted into the bottom of the cell, and a graphite anode, which is immersed into the molten material in the cell. In that known process, the molten salt mixture which contains lithium metal is withdrawn from the cell and the lithium metal is separated outside the cell. Because chlorine gas is evolved and the end of the cathode is formed like a venturi tube, a natural circulation is imparted to the molten material. A further reaction of lithium metal in the molten mixture is to be avoided.
Impurities of whatever kind are highly undesirable in the lithium metal if it is to be used in nuclear technology in the production of alloys and in lithium batteries.
For this reason it is known from U.S. Pat. No. 3,962,064 issued June 1976 to Brut et al in the production of high purity lithium metal to perform the fused-salt electrolysis in an electrolytic cell which has no diaphragm and in which the lithium metal which has separated is collected on the surface of the electrolyte and the electrolyte level is raised so that the metal is forced out of the cell through a system of overflows and is conducted to a receiver. The receiver contains a protective gas atmosphere, in which the liquid lithium metal having a purity of 99.9% is cast to form ingots. That known apparatus has the disadvantage that the equipment is expensive and that air is used in the known process as a pressure fluid for raising the level of the electrolyte (and of the lithium metal). Besides, the chlorine gas which is evolved is diluted with a large volume of air and is blown out to the cell together with this volume of air. This has the result that oxygen or air is inherently introduced into the system as an impurity, which is undesirable. Similarly, U.S. Pat. No. 4,724,055 issued Feb. 9, 1988 to Le Roux et al discloses the production of lithium metal using a diaphragmless electrolytic cell. This cell utilizes the chlorine gas produced to circulate the molten electrolyte and the further maintains an electrolyte temperature of less than 400.degree. C., and a chlorine gas temperature of less than 300.degree. C. to help prevent corrosion of the electrolytic cell container by the chlorine gas.
U.S. Pat. No. 4,455,202 issued Jun. 19, 1984 to Sintim-Damoa et al discloses a method of preventing chlorine gas production during lithium metal production in an electrolytic cell by using only lithium oxide instead of lithium chloride as a feed stock and by using a liquid metal cathode into which the metal lithium is absorbed. The liquid metal of the cathode containing the liquid metal lithium is then further processed to separate the lithium metal. This process has the commercial disadvantage of requiring costly preparation of lithium oxide from native lithium containing ores and the additional step of removing the lithium metal from the liquid metal of the cathode.
Another method of producing lithium metal by fused-salt electrolysis is disclosed in U.S. Pat. No. 4,740,279 issued Apr. 26, 1988 to Muller et al. In this reference the lithium metal is produced in a diaphragmless electrolytic cell in which the molten lithium metal is collected from the surface of the electrolyte by an annular trough and siphoned via a pipe to a separating chamber which communicates with the electrolytic cell and is sealed from the chlorine gas atmosphere in the electrolytic cell. An inert atmosphere is required to protect the lithium metal from reacting with air or chlorine.